Wifi Extender Problem

Hi there, I'm trying to help a neighbour with an wifi extender issue. She has one set up to get signal from the main wifi box in the house to her office outside. The extender seems to have hijacked the wifi signal and she can no longer get signal from in the house (where the main box is plugged in!). We've tried unplugging the extender plug in the outside office but could still not get signal from the main box inside. The extender is a Devolo brand one and I cannot see any help from their website regarding this although I've heard of this problem happening before. Does anyone have any advice as to what to do? I don't want to leave her with no wifi either! Many thanks

I do not understand what you mean by "hijacked." All a Range Extender does is connect to the router or wireless access point ("main wifi box") and rebroadcast the signals it receives from the access point or devices which connect to it.

It seems to think that the extender plug in the office is now the main router or wireless access point. So there is no wifi signal unless that particular extender plug is plugged in if that makes sense? And the main router seems redundant.